Justice-involved youth (JIY) aged 18-24 are at significant risk for HIV but are unlikely to know their HIV status. Rates of HIV in justice populations are 2-5 times higher than the general population, yet despite this risk, HIV testing in justice settings, particularly community supervision programs, is not universal. Even after identification, data also suggest that linkage to community HIV care in justice populations is much lower compared to general population rates, due to system/organization-, staff-, and individual-level factors, particularly youth substance use (SU). Overcoming barriers to SU screening and enrollment in SU care is, therefore, central to decreasing JIY?s negative HIV-related outcomes. Intensive efforts to increase screening and improve linkage to HIV (including PrEP for HIV? youth who are behaviorally eligible) and SU services for JIY are needed that address youth as well as justice and health/behavioral health system-level barriers. This project proposes to embed HIV testing outreach workers from a youth focused medical and HIV treatment program into and alternative sentencing program (ASP) to deliver a new service delivery model (Link2CARE) that integrates evidenced-based protocols for JIY to a) promote HIV and STI testing, HIV and SU risk screening and b) provide onsite intervention and c) cross-system linkage to HIV, STI and SU care. This is a first R01 proposal from proposed Principal Investigator with significant experience working within justice settings and supported by a strong multi-disciplinary team of senior researchers to achieve study aims. Guided by CFIR and Andersen?s Behavioral Model of Health Services Use, our Specific Aims are: 1) Among 450 JIY (18-24 y.o.), randomized to either Link2CARE or standard of care (SOC), to determine the efficacy of Link2CARE delivered by health staff embedded within the ASP, on (a) HIV outcomes: uptake of HIV testing /repeat testing; HIV risk behaviors; (b) STI outcomes: uptake of STI testing/repeat testing, linkage to care of JIY with STIs; (c) SU outcomes: SU screening, SU, and linkage to care of SU JIY; and (d) (exploratory) linkage to care of HIV+ JIY and HIV? JIY who are behaviorally eligible for PrEP; 2) to determine the influence of theoretically-based intervention mechanisms of change (e.g., predisposing characteristics, enabling resources, perceived need, organizational climate; staff attitudes) on the proposed HIV and SU outcomes; and 3) To describe Link2CARE implementation and elucidate the system/organizational-, staff-, and youth-level factors that influence implementation (i.e. acceptability, sustainability, feasibility) of Link2CARE in an ASP to develop a plan for dissemination and scale-up of Link2CARE in New York City. We propose a 2-phase study. In Phase 1: Adaptation for Link2CARE, we will adapt the intervention components for use among JIY and pilot the resulting protocols with n=8 JIY; and finalize the resulting Link2CARE intervention. In Phase 2: Link2CARE Efficacy Trial and Implementation Evaluation, we will test Link2CARE among N=450 JIY enrolled at the ASP (Phase 2a) and conduct process evaluations with healthcare staff and ASP staff (Phase 2b).